During the present year we have been able to quantify intermolecular forces involving several kinds of phospholipid bilayer membranes. The forces of interest act between membranes as well as between molecules in the same bilayer. There appears to be close analogies between forces involving lipids and those stabilizing aggregates of proteins and nucleic acids. The experimentally decaying "hydration forces" which dominate bilayer repulsion below 30 A separation are seen to depend, both in magnitude and decay rate, on the chemical identity of other groups on the bilayer surface. A rationale is now apparent for the activity of enzymes that modify these groups and reside on the bilayer surfaces of naturally occurring residues. Electrostatic forces are observed between bilayers charged by the adsorption of ions or by inclusion of charged phospholipid species. These forces extend beyond the 30 A reach of the hydration force. Their exponential decay is of the magnitude expected for electrostatic forces screened by ionic solutions.